GLENDALE, AZ - NOVEMBER 29: Runningback Brian Westbrook #20 of the San Francisco 49ers rushes the football against the Arizona Cardinals during the NFL game at the University of Phoenix Stadium on November 29, 2010 in Glendale, Arizona. The 49ers defeated the Cardinals 27-6.

Photo: Christian Petersen, Getty Images

GLENDALE, AZ - NOVEMBER 29: Runningback Brian Westbrook #20 of the...

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San Francisco 49ers' Anthony Dixon (24) celebrates his touchdown against the Arizona Cardinals as teammates Anthony Davis (76) and Barry Sims (65) look on during the first quarter of an NFL football game Monday, Nov. 29, 2010, in Glendale, Ariz.

Photo: Ross D. Franklin, AP

San Francisco 49ers' Anthony Dixon (24) celebrates his touchdown...

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San Francisco 49ers running back Brian Westbrook plays with a football at the 49ers NFL practice facility in Santa Clara, Calif., Wednesday, Dec. 1, 2010.

According to his pinpoint forecast, the storm system will come out of the backfield, move through the tackle box and spread across the green tundra of Lambeau Field.

"Yes, kind of thunder and lightning," the 49ers' head coach said Wednesday.

Singletary is referring to new starting running back Brian Westbrook as "lightning," the nine-year veteran with all the "spinning, the turning, the elusiveness."

"Thunder," then, would be backup running back Anthony Dixon, the rookie expected to hit the hole and any defender in his way.

So, how many running backs does it take to replace Pro Bowl centerpiece Frank Gore, who is out for the season with a fractured right hip?

In the 49ers' case, the answer will be two running backs who could not possibly be any more different.

"I would say I'm on one end of the spectrum and he's on the other," Westbrook said. "He's young. He's excited. He's full of something."

Dixon most certainly is. He's a rookie who went from homeless teenager to sixth-round draft pick. He laughs out loud. He smiles just because. When he's not delivering hits on special teams, he's smacking himself in the helmet for inspiration's sake.

Dixon is built like a Mac truck, if Mac trucks were 6-foot-1 and 233 pounds, but he runs like a sports car, if sports cars could zig and zag at the same time.

"Coach, he kind of tells me to tone it down a little bit," Dixon said, speaking like a rookie who has been told a hundred times to stop jitterbugging and get through the running lane.

"He wants me to be power, you know. I tell him all the time, 'I'm power, coach, but I dance well. I've got good feet, too.' "

That's true, but Singletary won't let Dixon have it both ways. When Singletary wants fresh moves, he'll put Westbrook on the floor.

He's a 31-year-old Pro Bowler with experience in running the ball, catching passes and overcoming surgeries and concussions.

Westbrook looks serious, and plays serious, too. He didn't smile when sitting on the bench for all but a handful of plays for the first 10 games, and he didn't go crazy after carrying the ball 23 times for 136 yards Monday when Gore went down.

He's hardened enough to know dancing in the backfield doesn't make plays happen. All it makes are 3-yard losses and dog-pile tackles.

"I've always been even-keeled," Westbrook said. "I don't go into games blowing snap bubbles and spittin' and yelling. I have a responsibility to do. I go hard every day at practice when I can. When I go out there in the games, I go out there prepared and ready to play."

The truth is, this odd couple needs each other as much as the 49ers need the duo.

Dixon is too unseasoned to be an every-down back. He hasn't mastered the pass-protection duties of his position. He still runs like lightning when that's not his job.

Westbrook, on the other hand, is too over-seasoned to be the bell cow of a run-heavy offense.

He missed practice Wednesday with a sore ankle after playing his first full game in a year. At this point, he is always one concussion away from calling it a season.

It has been a stormy season for the 49ers. As far as closing acts go, "thunder" and "lightning" are a perfect match.

"You know, that's been used a lot through the years with running backs," Singletary said. "With both those guys, it literally is applicable."